Paradoxical ventral tegmental area GABA signaling drives enhanced morphine reward after adolescent nicotine
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Background
An important yet poorly understood risk factor for opioid use disorder is adolescent nicotine use. We investigated the neural mechanisms underlying this understudied interaction.
Methods
Male and female adolescent mice received two-weeks of nicotine water (Adol Nic) or plain water (Adol Water). In adulthood, mice underwent three morphine tests: conditioned place preference (CPP), locomotor sensitization, and two-bottle choice. Ex vivo ventral tegmental area (VTA) brain slices were assessed via patch clamp for GABA and dopamine (DA) neuron morphine responses. Finally, VTA GABA neurons were chemogenetically inhibited during morphine CPP.
Results
In adulthood, Adol Nic mice had greater morphine CPP, more morphine locomotor sensitization, and more choice-based oral morphine consumption vs. Adol Water mice. In contrast, adult mice given nicotine vs. water had similar morphine CPP. Patch clamp analysis of VTA neurons from adult Adol Water mice showed canonical cell-type responses to bath-applied morphine: fewer action potentials in GABA neurons and more in DA neurons. Paradoxically, VTA GABA and DA neurons from adult Adol Nic mice did not show these morphine responses. In support of a causal relationship between GABA neuron firing and reward behavior, chemogenetic inhibition of VTA GABA neurons in Adol Water mice during pairing increased morphine CPP. In contrast, inhibition of VTA GABA neurons in Adol Nic mice brought morphine CPP down to control levels.
Conclusions
These data reveal an electrophysiological mechanism by which adolescent nicotine intake promotes morphine reward later in life, showing that adolescent nicotine exposure alters reward circuitry well into adulthood.
